1. Department of Environmental Science, Università degli Studi di Milano-Bicocca, Piazza della Scienza 1, I-20126 Milano, Italy; Laboratory for Physical Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland; and Physical Chemisty Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland